The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED display device comprises a plurality of pixels aligned in array. The pixel drive circuit is utilized to drive the organic light emitting diode to emit light. The driving method of the OLED display device has the analog driving method and the digital driving method. When the analog driving method is used, it will easily happen that the driving currents of various pixels are different under the same driving data signal voltage and result in the Mura because the differences exist among the property parameters of the thin film transistor elements of different pixels. However, the digital driving method is used, the appearance of the Mura can be effectively suppressed.
FIG. 1 shows a 3T1C pixel driving circuit used for an OLED display device according to prior art, comprising: a first thin film transistor T1, a second thin film transistor T2, a third thin film transistor T3, a storage capacitor Cst and an organic light emitting diode OLED. The second thin film transistor T2 is a drive thin film transistor, and a gate and a source of the second thin film transistor T2 are respectively coupled to the first node A, the second node S, and the first thin film transistor T1 is employed to charge the first node A, i.e. the gate of the second thin film transistor T2, and the third thin film transistor T3 is employed to discharge the first node A, i.e. the gate of the second thin film transistor T2.
As performing digital driving to the aforesaid 3T1C pixel driving circuit used for the OLED display device, the first thin film transistor T1 charges the first node A, and the third thin film transistor T3 discharges the first node A, and thus, the first node A, i.e. the gate of the second thin film transistor T2 only outputs two Gamma voltage levels: the highest Gamma voltage (GM1) making the organic light emitting diode brightest, and the lowest Gamma voltage level (GM9) making the organic light emitting diode darkest. According the formula of calculating the current flowing through the organic light emitting diode OLED:I=k(VGS−Vth)2=k(VA−VS−Vth)2 
wherein k is an intrinsic conductive factor of the drive thin film transistor, i.e. the second thin film transistor T2, and VGS is a gate-source voltage of the second thin film transistor T2, and Vth is a threshold voltage of the second thin film transistor T2, and VA is the voltage of the first node A, i.e. a gate voltage of the second thin film transistor T2, and VS is a voltage of the second node S, i.e. a source voltage of the second thin film transistor T2.
The voltage VA of the first node A making the organic light emitting diode brightest is the highest Gamma voltage (GM1), and the degeneration or the inconsistency of the thin film transistor elements result in that the variation of the threshold voltage Vth is smaller relative to the variation of (VA−Vs). In comparison with the analog driving method, the digital driving method can suppress the Mura of the OLED display device.
With that the first thin film transistor T1 charges the first node A, and the third thin film transistor T3 discharges the first node A, the first node A is ultimately controlled to output only two Gamma voltage levels. The OLED display device performs the brightness modulation with a way similar to the Pulse-Width Modulation (PWM) for cutting the gray scales. As shown in FIG. 2, driving the 6 bits OLED display device is illustrated. Each frame of image is divided into six Sub frames according to an order of display times. By controlling the charge, discharge times of the Sub frames with combination of the sense of the human eyes to the brightness, which is the integration principle in time. Two Gamma voltages (i.e. GM1 and GM 9) can be utilized to show the images of various gray scale brightnesses and to control the color components outputted by various Sub frames. As shown in FIG. 2, the output order of the color components from the first Sub frame to the sixth Sub frame is from bit6 to bit1, wherein the gray scale corresponded with the color component bit6 is the highest, and the gray scale corresponded with the color component bit1 is the lowest.
FIG. 3 shows that in the digital driving method according to prior art, the diagram that the 6 bits OLED display device continuously shows a plurality of frames of image. Each frame of image is divided into six Sub frames, and the corresponding times of all the Sub frames are equal. The output orders of the color components of each frame of image are the same. As shown in FIG. 3, all the output order of the color components from the first Sub frame to the sixth Sub frame of the (N−1)th, the Nth and the (N+1)th frames of image are bit6 to bit1.
The advantage of the driving method is that the sizes of the six Sub frames corresponded with each frame of image are the same, and the color components are outputted in the same order. The driving is easy to be achieved. The shortcoming is that the different integral effects generate to the two adjacent frames of images because the data signals are different. For example, the color component bit3, the color component bit2 and the color component bit1 respectively outputted by the fourth Sub frame to the sixth Sub frame in the (N−1)th frame of image will generate new integral effects, which are different from the color component bit6, the color component bit5 and the color component bit4 respectively outputted by the first Sub frame to the third Sub frame in the Nth frame of image because the data signals are different. Accordingly, the image flicker occurs and the gray scales in sequence show ladder reforms, and the display effect is not right.